This invention relates to engraving heads of the general type disclosed in Buechler U.S. Pat. No. 4,450,486. Such engraving heads comprise a diamond stylus carried by a holder mounted on an arm projecting from a torsionally oscillated shaft. A sine wave driving signal is applied to a pair of opposed electromagnets to rotate the shaft through a maximum arc of approximately 0.25 deg. at a frequency in the neighborhood of about 3,000 to 5,000 Hz.
A guide shoe is mounted on the engraving head in a precisely known position relative to the oscillating stylus. The engraving head is supported for tilting movement by a set of leaf springs secured to a rearwardly projecting bar. A DC motor rotates the bar so as to bring the guide shoe into contact with a printing cylinder to be engraved. When the guide shoe is in contact with the printing cylinder, the stylus oscillates from a position just barely touching the printing cylinder to a retracted position about 100 microns distant from the surface of the cylinder.
Once the guide shoe is in contact against the printing cylinder a video signal is added to the sine wave driving signal for urging the oscillating stylus into contact with the printing cylinder thereby engraving a series of controlled depth cells in the surface thereof. The printing cylinder rotates in synchronism with the oscillating movement of the stylus while a lead screw arrangement produces axial movement of the engraving head so that the engraving head comes into engraving contact with the entire printing surface of the printing cylinder.
In engraving systems of the type taught by Buechler, it is necessary for the machine operator to perform a tedious trial and error setup procedure at one end of the printing cylinder prior to commencement of engraving. This procedure involves adjustment of the gain on amplifiers for the sine wave driving signal and the video signal so as to produce xe2x80x9cblackxe2x80x9d printing cells of a desired depth together with connecting channels of another desired depth and clean non-engraved white cells. Each change of one of the control variables interacts with the others, and therefore the setup becomes an iterative process.
It is therefore seen that a need has existed for an engraving system which may be quickly and easily set up to engrave cells of precisely controlled dimensions in the surface of a gravure printing cylinder.
The present invention provides an engraving apparatus and method wherein a plurality of parameter signals are supplied to a setup circuit for computing engraving parameters to control the engraving response of the engraving stylus to an input video signal. In a preferred embodiment an input AC signal and an input video signal are applied to a multiplication circuit wherein they are multiplied by multiplication factors which are generated by a setup circuit. The output signals from the multiplication circuits are combined with a white offset signal to produce an engraving signal for driving the engraving stylus to engrave a series of cells of the desired geometry.
The setup circuit is provided with input signals which indicate a desired black cell width, a desired channel width, a desired highlight cell width and the video voltage level at which a highlight cell of the desired width is to be engraved. Further, in accordance with the present invention the setup circuit may be provided with an input signal which adjusts the multiplication factor as appropriate for the geometry of the particular stylus tip which happens to be in use. This input signal corresponds to the cutting depth/width ratio, which in turn depends upon the tip angle of the stylus.